COLOUR DYNAlVHC REQUIREMENTS AND THE COLOROID SYSTEM By A. NE;\ICSICS Department of Drawing and Composition, Technical University, Budapest Received: July 17, 1978 Introduction For the architect dcsigning coloured enTironment, colour may be botH a technical and an artistic means. In the first case the possibility to define technical parameters assigned to different colours, in the latter case, to express the compositional relations between colours by numbers requires to identify each member of the group of colours by indices. Both requirements relate the prohlem of colour notation to that of colour systematization. The relation between the millions of distinct colours in the set and their indices cannot be an accidental one, hut has to rely on colour systematization. Many colour experts couple the concept of colour systematization almost entircly to colorimetry ([1] to [5]). According to the well-known wording hy G. "",VX-SZECKI [6], colorimetry is a means for predicting if t·wo yisual stimuli of different 8pectral distribution will produce the same colour sensation uncleI' giyen conditions, by predicting the place of the two yisual stimuli in a giyen colour space. If the colour-space co-ordinates of OIle stimulus equal those of the other one, a person with normal colour yision will feel the two colours to be equal. This means is increasingly applied in modern industry, among others for numerically settling the difference hetween two colours. The endeavour to express the rate of change in colour perception by a proportionalnumcrical change of colour indices has actually becomc the most important criterion of qualifying the colour systcms. For colour dynamics, the scicnce of colour spacc design, to possess colour indices for numcrically descrihing colour compositions has become a technical necessity hy no·w, changing, however, the requirements for colour systems. This paper is going to deal with thcse new requirements as well as with the COLOROID colour system elaborated to meet them. ReseaTeh work in connection with the COLOROID coluur system has been carried on since 1962 at the Technical University, Budapest. The work 'was s1 :ll'ted by creating differcnt aesthetically~ uniform psycho-mctric scales, that 3* 36 NEMCSICS is, those felt to be uniformly varying when considered as a whole. "Considered as a whole" means that test subjects rated e.g. the brightness scale of about 20 to 100 members from white to black by viewing it at once, without con­ centrating on the relation between neighbouring colours in a section of the scale. The number of colours between each two colours of the scale was evenly increased up to a density whele two adjacent colours could not be used any­ more in a colour harmony design. The difference between these colours was named harmony interval. The number of harmony intervals between two neigh­ bouring members of aesthetically uniform scales was found everywhere equal, contrary to the perception ally even steps. Scoring by over 70 thousand persons was used when creating the scale. Most of our results have been published successively [7-21]. During this work, our conception of the colour space of this colour dynamic system has been gradually modified. Thus, the set of our publications does not sho·w the details but the different phases of development of the COLOROID. This is the first attempt to present in full the experiments made for creating the system, its interrelations and their application for colour composition. 1. The requirements of colour dynamics for colour systematization The basic problem of colour systematization is rooted in the subjective colour perception, which is the result of highly compounded effects. It depends on the spectral energy distribution of the light source illuminating the coloured surface to be observed, on the luminance factor of the surface, on the geometry of observation and reflexion. Thus, in order to unambiguously express a colour perception, the spectral energy distribution of the light source, the luminance factor of the surface to be observed, and the geometry of observation and illu­ mination have to be fixed. Also the characteristics of the colour perception mechanism influence the colour sensation. Estimation of a colour is also affected by factors such as the chromatic adaptation, the chromatic constancy, the phenomenon of colour contrast and the fatigue of the eyes. Therefore, in defining a colour also its environment, the duration of observation, the state of the chromatic adaptation have to be fixed. Besides, the colour sensation depends on the age, temperament, educa­ tion, etc. of the viewer, therefore colour sensation grades can only be formed by statistically averaging the opinion of a great number of observers. In the following we intend to enumerate the requirements of colour dynamics poi 1.ting beyond the general problems of colour systematization. Both the discussion and the conclusion will be facilitated by being definitely referred to the Munsell and the COLOROID colour systems. COLOROID 37 1.1 Indices expressing the three characteristics of visual sensation: hue saturation and brightness In different colour systems, colour indices are assigned to different num­ bers of colour samples. In the Munsell and COLOROID colour systems, simil­ arly to other colour systems, these colour indices signify not only the place of the sample within the collection but also the relative values of the three char­ acteristics of visual sensation: hue, saturation and brightness. This is very important for colour dynamics. In everyday life we remember or describe a colour by these three qualities. Any difference or similarity in them induces us to speak of a colour different from or similar to another colour. Also aesthetical decisions, e.g. whether a colour composition is harmonious or disharmonious are made according to differences felt bet"ween colour char­ acteristics. 1.2 Indices expressing the aesthetical continuity of the colour space Colour sensations are not physical quantities to be characterized by physical units, but to be expressed by the change of the three colour sensation qualities in a given numerical range in both colour systems. Each colour sensation series consisting of colours evenly changing in relation to a certain quality is expressed by different colour index series in the two colour systems. Let us compare them. The two colour sensation indices can only be compared by relating indices assigned to colour sensations produced by defined stimuli. Brightnesses and saturations in the two colour systems are related by: Vc = 10 V1.2219Vm - 0.23111 V~ + 0.23951 Pm - 0.021009V~ + 0.0008404V~ and 3 T = ab VC 2 respectively, where Vc and V m express COLOROID and lVIunsell brightnesses, T is saturation in the COLOROID system, C the lVIunsell-chroma; a and b in the second formula mean that the assignment of both COLOROID and lY.lunsell indices to the saturation of a colour depends also on its brightness and hue. The formulae show that to sensations elicited by identically changing stimuli, a set of indices changing according to a different law is assigned in each system. Thus, the two systems suggest different ways of measuring the colour sensation. Let us compare the two suggestions to see which one suits better the requirements of colour dynamics. 38 iYE}lCSICS In the ]Hu1Zsell system a colour series evenly changing from the aspect of a certdn parameter was developed with approximately equal, small colour differences between its members what meallS that between each index approx­ imately the same number of shades can be distinguished. This characteristic of the i1;lullseiZ system for describing colour differences has become increasingly important with the development of colour measurement. It is undoubtedly important also for colour dynamics to fix ho'w much the actual colour may be aIlo'wed to differ from the planned one. However, the particular colour dynamic prohlems arise beyond this field, referring to colour-compositional relationships to he fixed by colour indices. The colours of our environment belong to various parts of the colour space. Therefore the planning of a coloured environment has to bring about harmony of hue, saturation and hrightness between highly different colours. This is -why far greater importance is due to the aesthetical evenness of the whole colour space than to the reliable equality of small colour differences. The endeavours of colour systematization resulting in the present form of the klullsell colour system produced psycho-physical scales fairly approximating the ability of the human eye to distinguish colours in various ranges hut little suiting aesthetical applications. In our experiments to be presented later, we found that in the klu1Zsell system the brightness gradation of the colours -was denser for dark colours than 1\-ould he required byaesthetical evenness. In spite of the almost equal colour differences, the saturation steps were found to be aesthetically denser in dark areas of the colour solid than in bright ranges. Furthermore, the jHu1Zsell chroma steps were found in the highly saturated fields to be aesthetically far too scarce, whereas in the unsaturatcd fields far too dense. To be concise, according to the second requirement of colour dynamics for colour systems, the index yariation IW5 to follow an aestlwtically eyen yariation of colour5. This means that the requiremcnts of colour dynamics arc met hy a colour space built on ..,esti1l'ticdly equal colour cliffCTellccs, i.e. where large (rather than small) colour dif[(TenCe5 are equal. The colour "pace of COLOROID has been elahorated :::ceordingly. 1.3 VisuaZi:;ing the colour by indices Two colours and their mixing rates are glyen, the colour re:oulting from their mixing has to he determined. In colour ellyironm.ent design this task is quite frequent, e.g. when a third, harmonious colour is looked for to match t-wo giyen colours. The indices in the Ilill1Zsell system aTe of no help, they being 1Il no direct relationship to the colour stimuli, phY3ical quantities eliciting colour sensation.